Problem: $ E = \left[\begin{array}{rr}4 & 2 \\ -2 & -1\end{array}\right]$ $ D = \left[\begin{array}{rrr}5 & -2 & 0 \\ 0 & 1 & -1\end{array}\right]$ What is $ E D$ ?
Answer: Because $ E$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ E D = \left[\begin{array}{rr}{4} & {2} \\ {-2} & {-1}\end{array}\right] \left[\begin{array}{rrr}{5} & \color{#DF0030}{-2} & \color{#9D38BD}{0} \\ {0} & \color{#DF0030}{1} & \color{#9D38BD}{-1}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ E$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ E$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ E$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{4}\cdot{5}+{2}\cdot{0} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ E$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{4}\cdot{5}+{2}\cdot{0} & ? & ? \\ {-2}\cdot{5}+{-1}\cdot{0} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ E$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{4}\cdot{5}+{2}\cdot{0} & {4}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{1} & ? \\ {-2}\cdot{5}+{-1}\cdot{0} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{4}\cdot{5}+{2}\cdot{0} & {4}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{1} & {4}\cdot\color{#9D38BD}{0}+{2}\cdot\color{#9D38BD}{-1} \\ {-2}\cdot{5}+{-1}\cdot{0} & {-2}\cdot\color{#DF0030}{-2}+{-1}\cdot\color{#DF0030}{1} & {-2}\cdot\color{#9D38BD}{0}+{-1}\cdot\color{#9D38BD}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}20 & -6 & -2 \\ -10 & 3 & 1\end{array}\right] $